The present invention relates to an image forming apparatus including an image transfer carrying means having an endless sleeve-like shape and having flexibility, a plurality of image forming means which are arranged along the image transfer carrying means such that respective image carriers thereof are in contact with the image transfer carrying means, each image forming means comprising a latent image forming means for forming a latent image on the image carrier and a developing means for developing the latent image formed on the image carrier, and transfer bias applying means which are disposed on the back of the image transfer carrying means at positions where the respective image carriers are in contact with the image transfer carrying means for applying transfer bias, whereby toner images developed by the image forming means are sequentially transferred to the image transfer carrying means such that the toner images are superposed on each other.
Tandem-type image forming apparatuses are categorized into two types as:
an apparatus employing a paper delivery method which comprises a plurality of image forming stations arranged in an array, in which a receiving medium is electrostatically attracted to a delivery belt and is fed to be brought in contact with the respective stations in order and electrostatic transferring force is applied between each station and the receiving medium, thereby superposing toner images of plural colors while directly transferring the toner images to the receiving medium; and
an apparatus employing an intermediate transfer method which comprises a plurality of image forming stations arranged in an array, in which an intermediate transfer belt made of a dielectric substance is fed to be brought in contact with the respective stations and electrostatic transferring force is applied between each station and the intermediate transfer belt so as to transfer primarily toner images of the respective stations one by one to superpose the toner images on the intermediate transfer belt and the superposed toner images are transferred secondarily from the intermediate transfer belt to a receiving medium at once.
In the aforementioned paper delivery method, it is required to provide a means (roller or brush) for attracting the receiving medium to the delivery belt and high voltage power supply. In the intermediate transfer method, however, such a means and high voltage power supply are not required. Further, in the paper delivery method, it is required to strictly control the transfer bias to be applied to respective image transfer portions according to the size, the thickness, and the kind of the receiving medium. In the intermediate transfer method, the primary transfer of toner images is conducted to the intermediate transfer belt of which resistance, thickness, and surface roughness are constant regardless of the aforementioned factors of the receiving medium. The control of the transfer condition including the transfer voltage or transfer current and contact pressure must be conducted only for the secondary transfer of the toner images to the receiving medium. Therefore, the intermediate transfer method has a lot of advantages.
On the other hand, the apparatus can also be categorized according to the arrangement of the respective image forming stations. There are a method of arranging the stations horizontally and a method of arranging the stations vertically. The former has a disadvantage of requiring a larger area for placing, while the latter has a disadvantage of making the apparatus too tall to be put on a desk.
Therefore, a method of arranging the respective image forming stations obliquely is conventionally known as disclosed in Japanese Patent Unexamined Publication No. H11-95520 and Japanese Patent Unexamined Publication No. H8-305115. The former has exposure devices corresponding to the image forming stations, respectively, and the latter has an exposure device common to the respective image forming stations.
In an image forming apparatus employing an image transfer carrying means composed of a carrying belt or an intermediate transfer belt which is in contact with a plurality of image carriers to sequentially receive toner images from the image carriers to form a multiple-color image thereon and carries the multiple-color image, however, there are frequently differences between the condition of transferring a first toner image from the first image carrier and the condition of transferring a second toner image from the second image carrier onto the first toner image, . . . and the condition of transferring a n-th toner image from the n-th image carrier onto the n-1-th toner image. The condition may be changed because the condition is affected by attributes of the toner image(s) previously transferred. In particular, the contact pressure and contact form between each image carrier and the image transfer carrying means, the form of applying transfer bias for transferring each toner image and the like are important requirements for determining the condition for sequentially transferring the toner images by contacts of the plurality of image carriers such that the toner images are superposed on each other.
FIG. 1 is an illustration for explaining the phenomenon of image deterioration due to the transfer bias at a nip. Typically, a conductive roller 16 made of an elastic material such as rubber is pressed against an image transfer carrying means 18 with a contact pressure ƒ as shown in FIG. 1 so as to form a nip between an image transfer carrying means and an image carrier carrying a toner image formed thereon. Therefore, the transfer nip is formed between the image transfer carrying means 18 and the image carrier 17 and a transfer bias is applied to the nip, thereby transferring the toner image T from the image carrier 17 to the image transfer carrying means 18. During this, the transfer current flowing from the elastic conductive roller 16 through the image transfer carrying means 18 is substantially constant over the entire area of the nip because the lengths of current paths from the contact between the elastic conductive roller 16 and the image transfer carrying means 18 to the image carrier 17 are all constant.
However, since there are gradually increasing spaces at the entrance end and the exit end of the nip of the image transfer carrying means 18, the discharge phenomenon occurs at the spaces with higher voltage of the transfer bias applied from the conductive roller 16, causing undesirable phenomena such as toner T scattering from predetermined positions and thus leading to image deterioration.